Constant-air-volume systems

O

Use in new systems discouraged by code.

+Q +P — Q +W +Q

	R
0	/M
	———— XH1

Winter heating

Summer:

Single mixing with room air:

O + R => M (cooling + dehumidification) => C (re-heat) => I (Q + W ) => R Double mixing with room air:

O + R => Ml (cooling + dehumidification) => C’ + R => M2 or I (Q + W ) => R Winter:

O (pre-heat) => HI + R => M (humidification) => H2 (re-heat) => I (Q + W) => R

4.3.1 Variable-air-volume systems

R

Example:

You turn the fan speed up or down in your car. Q = m (iR — ii)

O

AHU fan varies power to match loads. Less load => lower fan power.

Pressure in supply ducts is maintained to a fixed value.

Design cooling:

• box is 100% open

• no reheat

Off-design cooling:

• zone temperature drops since cooling load decreases

• box throttles until minimum flow is reached

Dead band:

• no control action

• start reheat at lower limit

Off-design heating:

• minimum primary air

• thermostat increases reheat as space temperature falls

Design heating:

• fully energized

FIGURE 11.23 Vanable-air-volume system with optional reheat.

VAV terminals:

• Single-blade dumper (pressure dependent or independent)

• Air valve

• Induction

Primary flow induces secondary flow from plenum.

Fully

(i)

KIGl’RE 17.3 VAV boxes: fa) single-blade. pressure-dependent: (bl single-blade, pressure-»dependent:

:rvJ I Cl :l«r valve.

FIGURE 11.246 Schematic diagrams of series and parallel fan-powered VAV box.

Cutaway drawing of parallel VAV box. (Courtesy of Titus, inc.)

{ (<*)

<b)

FIGURE 11.25 (a) Flow characteristics for series fan-powered VAV boxes. Cross-sectioned areas represent primary airflow. Total box airflow is constant. (*) Flow characteristics for parallel fan-powered VAV boxes. Cross-hatched areas represent primary airflow. Total box airflow is higher during cooling than healing. Fan operates only at zone temperatures below the adjustable set point.

(Optional) reheat Induction

FIGURE 11.24л Schematic diagram of induction VAV box.

Fan — powered series: Fan is always on and space flow is constant. Damper controls supply of primary air. Perimeter zones may need baseboard or fan-coil units.

Fan-powered parallel: Fan injects plenum air to reheat. Supply pressure drives primary flow that is controlled by dumpers. Variable space flow => less fan energy.

Advantages of VAV: O

Disadvantages and problems of VAV for off design (low flow rate): O

Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning